The invention relates to processing a spread spectrum signal.
In wireless systems, information typically is transmitted by modulating the information onto carrier waves having frequencies that lie within preassigned frequency bands. Radio frequency (RF) receivers demodulate the carrier waves to recover the transmitted information.
Spread spectrum communication systems spread transmitted signals over bandwidths much larger than those actually required to transmit the information. Spreading a signal over a wide spectrum has several advantages, including reducing the effects of narrow band noise on the signal and, in many situations, providing increased protection against interception by unwanted third parties. In a direct sequence spread spectrum (DSSS) system, the bandwidth of a transmitted signal is increased by modulating the signal onto a known pseudo-noise (PN) signal before modulating onto the carrier wave. The PN signal typically is a digital signal having an approximately equal number of high and low bits (or "chips"), which maximizes the spectrum over which the signal is spread. A typical implementation of a DSSS receiver recovers the transmitted information by demodulating the carrier wave and then multiplying the resulting signal with a local replica of the PN signal to eliminate the PN signal. The DSSS technique offers heightened security because the receiver must know the PN sequence used in the transmission to recover the transmitted information efficiently. Other spread spectrum techniques include frequency hopped spread spectrum (FHSS).
A DSSS receiver must be tuned to the carrier frequency of the signal to be received. In many systems (e.g., GPS and CDMA cellular telephony) the received signal is continuous or of long duration, so that it is practical for the receiver to use narrow-bandwidth phase-lock techniques to track the carrier frequency of the incoming signal. But in a data collection system, such as might be used for wireless meter reading, the incoming signal is typically a short packet whose carrier frequency is subject to considerable uncertainty due to the low cost and simplicity of the transmitter. In such a case, phase-lock techniques become difficult, and the wider bandwidths required for fast acquisition render them less advantageous due to increased noise.
Receivers in a short-packet system where phase-lock techniques are impractical and where frequency tracking is required must somehow measure the frequency of the incoming signal and gain the information needed to tune onto the carrier frequency. Typically, such receivers use a frequency discriminator to measure the carrier frequency of the incoming signal. Commonly-used analog discriminator techniques include delay lines and stagger-tune detectors, while digital techniques include dual differentiators and arctangent algorithms. However, if a system is otherwise designed only to measure and report the magnitude response to DSSS signals of a particular code phase and frequency, each of these frequency measurement techniques requires the addition of system hardware that is dedicated to the task of frequency discrimination.